The present invention relates to a method of forming an oxidation resistant diamond layer or coating on a substrate and also to a method of forming a diamond layer on a cutting tool substrate material wherein nucleation of diamond on the substrate as well as adherence of the diamond layer or coating on the substrate are substantially improved.
As a result of many promising applications for thin diamond layers or coatings in the fabrication of wear resistant, protective coatings, optical coatings, electronic devices, etc., considerable effort has been expended in developing low pressure, low temperature processes for the deposition of these layers on various substrates, especially nondiamond substrates that would be involved in such applications. Representative of such low pressure thin diamond film deposition methods are chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), hot filament chemical vapor deposition (HFCVD), oxygen-acetylene torch, and plasma torch techniques.
The success of chemical vapor deposition processes in depositing polycrystalline diamond layers has opened the way for potential application of the layers as protective coatings on a variety of substrate materials. For example, it is known that diamond films on steel surfaces can successfully resist corrosion, see T. P. Ong and R. P. H. Chang, Appl. Phys.Lett., 58, Jan. 28, 1991. Diamond films have also been shown to be good optical coating materials which can resist mechanical wear, see T. P. Ong and R. P. H. Chang, Appl. Phys.Lett., 55, 1989.
However, there are instances where diamond layers or coatings will be subjected to an oxygen-containing environment at elevated temperature. In such an environment, the diamond film can be readily oxidized (or vaporized) as indicated by A. Joshi et al. in J. Vac. Sci. Technol., A8, 1990, and can oxidize much more readily compared to graphite as indicated by C. Johnson et al. in Mater. Res. Bull., 24, 1990. Diamond films have been found to require more energy to oxidize compared to bulk diamond with comparable defect density, R. R. Nimmagadda et al., J. Mater. Res. 5, 1990.
Patterson et al. in Mat. Soc. Symp. Proc., volume 140, 1989, have shown that diamond powder (or grit), single crystal diamond as well as chemical vapor deposited diamond films can be fluorinated. They found that in most instances the fluorine is attached to the external surface of the diamond. They also found that single crystal diamond slabs having such surface fluorination exhibit enhanced resistance to further oxidation.
In view of the potential application of chemical vapor deposited polycrystalline diamond layers or coatings for use on substrates in oxygen-bearing environments at elevated temperature (e.g. electronics, aerospace, cutting tool and other applications), a method for improving the oxidation resistance of such diamond layers or coatings would be desirable. Moreover, a method to improve the oxidation resistance of such diamond layers or coatings that can be incorporated in-situ in the diamond deposition operation would be desirable from a production standpoint to produce polycrystalline diamond coated substrates.
A severe drawback experienced to-date in practicing chemical vapor deposition processes for coating substrates with polycrystalline diamond has involved the need to pretreat nondiamond substrates in a manner to provide a sufficient density of diamond nucleation sites on the substrate surface to enable subsequent growth of a continuous diamond layer or coating. The most common pretreatment developed to-date to achieve the required diamond nucleation density involves abrading the substrate surface with diamond powder prior to conducting the low pressure deposition process. The Iijima et al technical article xe2x80x9cEarly Formation of Chemical Vapor Deposition Diamond Filmsxe2x80x9d, Appl. Phys. Lett., 57, p. 2646 (1990) attributes the efficacy of the diamond powder abrading pretreatment in enhancing diamond crystallite nucleation on the substrate to the seeding action provided by residual xe2x80x9cdiamond dustxe2x80x9d particles present on the substrate surface following the pretreatment. Unfortunately, the diamond powder abrading pretreatment constitutes a severe processing limitation for many potential applications where the diamond layer will be nucleated and grown on a nondiamond substrate and where large and/or non-planar substrate surfaces are involved.
Other pretreatments for enhancing diamond nucleation on nondiamond substrates have involved application of pump oil, diamond-like carbon films (DLC), and even fingerprints to the substrate surface. In addition, DC voltage biasing of the substrate has been employed to this same end. However, none of these techniques has been reported as providing significant nucleation enhancement during the thin diamond film deposition processes involved.
The potential applications of chemical vapor deposited polycrystalline diamond films or coatings in oxygen-bearing environments at elevated temperature (e.g. electronics, aerospace, cutting tool and other applications) in all likelihood will involve a wide variety of different substrate materials depending on the particular service application involved. Among possible substrate materials, so-called cemented carbide cutting tool materials appear to offer great potential for diamond coating to improve cutting tool performance. Such cemented carbide cutting tool substrates typically comprise a metal (e.g. cobalt) matrix and hard refractory metal carbide (e.g. tungsten carbide) particles dispersed in the metal matrix.
A method for improving the nucleation of polycrystalline diamond on such cemented carbide as well as other cutting tool substrates without the need for pretreating the substrate using the diamond powder abrading technique would be highly desirable.
A problem of poor adhesion of polycrystalline diamond layers or coatings on cemented carbide cutting tool substrates, such as tungsten carbide (WC/Co) has been experienced to-date and attracted the attention of prior art workers. For example, recent work on diamond coating of cemented carbide cutting tools has focused on 1) removal of near-surface cobalt by chemical etching (K. Shibuki et al., Surf. Coat. Tech., 36, 1988) and plasma etching (Y. Saito et al., J. Mat. Sci., 26, 1991) to allow diamond deposition to proceed and 2) improvement of adhesion of the diamond coating to the substrate. Adhesion strength has been accomplished by diamond seeding ( T. Y. Yen et al., Mat. Res. Soc. Symp. Proc., 168, 1990 and C. T. Kuo et al., J. Mat. Res., 5, 1990) and by pre-decarburizaton of the tungsten carbide (Takatsu et al., Mat. Sci. Enq., 140, 1991). These methods are said to increase the nucleation density of diamond, thereby improving the bond between the substrate and the coating. Explanations for the poor adhesion of diamond coatings on cutting tools have been proposed by several workers in the art; namely, S. Soderberg et al., Vacuum, 41, 1990, and P. R. Chalker et al., Mat. Sci. Eng. A, 140, 1991.
A method for improving the adhesion of polycrystalline diamond layers or coatings on cemented carbide and other tool substrate materials to improve coating adhesion to the substrate also would be highly desirable.
An object of the present invention is to provide a method for improving the oxidation resistance of chemical vapor deposited polycrystalline diamond layers or coatings on substrates by incorporating fluorine in the diamond layers during diamond nucleation and growth during the deposition process.
Another object of the present invention is to provide a method for improving the nucleation and growth of polycrystalline diamond on tool substrate materials, such as cemented carbide materials, without the need for pretreating the substrate using the diamond powder abrading technique.
Still another object of the invention is to provide a method for improving the adhesion of polycrystalline diamond layers or coatings on tool substrate materials, such as cemented carbide materials.
The present invention involves a method of forming on a substrate a diamond layer having improved oxidation resistance wherein a deposition medium (e.g. a plasma or flame) is formed from a gaseous source of carbon and fluorine such as, for example, a fluorocarbon in hydrogen, and the substrate and the deposition medium are contacted to deposit a diamond layer including fluorine dispersed throughout the layer in an amount effective to improve the oxidation resistance of the layer at elevated temperature. Preferably, the fluorine is incorporated in a polycrystalline diamond layer at least in an amount corresponding to the internal defect density (e.g. internal voids, grain boundaries and dislocations) of the deposited diamond layer.
In one embodiment of the invention, the gaseous source comprises hydrogen carrier gas and a fluorocarbon. The gas preferably comprises at least about 3 mass % carbon tetrafluoride as the fluorocarbon.
The invention also involves a method of forming a diamond layer on a substrate comprising a metal matrix and relatively hard particles disposed in said matrix, wherein the metal matrix and the hard particles of the substrate both are chemically etched, a deposition medium is formed from a gaseous source of carbon and fluorine, and the etched substrate and the deposition medium are contacted to deposit a diamond layer on the substrate.
In one embodiment of the invention, the metal matrix and the hard particles are chemically etched by contacting the substrate with one of the matrix or hard particle etchant and then with the other of the matrix or hard particle etchant. In one particular embodiment of the invention, the matrix etchant comprises a ferric chloride solution and the hard particle etchant comprises a potassium/iron cyanide solution.
In still another embodiment of the invention, the substrate comprises a cutting tool, for example, wherein the metal matrix comprises cobalt and the hard particles comprise tungsten carbide.
A particular embodiment of the invention involves a method of forming a diamond layer on a tool substrate comprising a metal matrix and relatively hard particles disposed in the matrix wherein the metal matrix and the hard particles of the tool substrate are chemically etched by a matrix etchant and a particle etchant, a deposition medium (e.g. plasma or flame) is formed from a carrier gas (e.g. hydrogen) including fluorocarbon, and the etched tool substrate and the medium are contacted to deposit a diamond layer on the tool substrate. The nucleation and growth of the diamond on the substrate as well as diamond coating adhesion are significantly improved by the etching of both substrate components. A polycrystalline diamond layer having a thickness of about 3 to about 4 microns preferably is formed.
The present invention also involves an article comprising a substrate having a layer of diamond deposited thereon wherein the diamond layer includes fluorine dispersed therethrough. The fluorine preferably is present in a polycrystalline diamond layer in an amount to improve the oxidation resistance of the diamond layer at elevated temperature. The substrate may comprise a chemically etched metal matrix and relatively hard particles in the matrix; for example, the matrix may comprise cobalt and the hard particles may comprise tungsten carbide particles of a cutting tool substrate.
The aforementioned and other objects and advantages of the invention will become more apparent from the following detailed descriptions and the drawings which follow.